Parkinson's disease is the second most common neurodegenerative disorder and affects over 1 million people in North America.2 The pathological process, degeneration of the dopaminergic neurons in the substantial nigra, causes profound depletion of striatal dopamine and motor impairment. This insight led to the introduction of L-dopa as a dopamine-replacement treatment for PD.3 Today, L-dopa continues to be the “Gold Standard” treatment for the motor symptoms of PD.4,5 Despite the considerable symptomatic relief it affords, long-term treatment with L-dopa has major limitations.6 After five to ten years of treatment with L-dopa, up to 60% of patients experience loss of L-dopa effectiveness and some debilitating complications,7,8 notably, an “on” and “off” motor fluctuation and involuntary choreic or dystonic movements, dyskinesia. This has become the limiting factor in management of patients in the later stages of PD.9 The development of dyskinesia might reflect desensitization of dopamine receptors.10 Most importantly, there is no clear evidence that L-dopa slows or halts the degeneration of dopaminergic neurons. In fact, in vitro cell culture studies suggested that dopamine and its oxidative metabolites are toxic to dopaminergic neurons, and raised the concern that L-dopa may actually accelerate the degeneration of dopaminergic neurons. Because of this concern, many clinicians avoid prescribing L-dopa early in the course of PD.11 
These major limitations of L-dopa therapy are linked to the activation of dopamine receptors. This has prompted a search for alternative treatment for PD not targeting the dopaminergic system.12 Striatal neuromodulators and transmitters other than dopamine are increasingly appreciated as critical regulators of motor function and offer new therapeutic opportunities to complement dopamine-replacement.
Over the last 10 years, the A2A adenosine receptor (A2AAR) has received increasing attention as a treatment for PD.13,14 This contention is based on our understanding of the role of the A2AAR in the basal ganglia and on the recent development of new, more selective A2AAR antagonists. Anatomical, neurochemical and behavioral evidence of adenosine-dopamine interactions underlie this new therapeutic approach.15-17 Anatomically, A2AAR density is high in the striatum, where receptor mRNA is co-expressed with D2 receptor mRNA in the striatopallidal neurons.18-20 This unique cellular distribution of A2A receptors suggests that A2A receptor antagonists can selectively modulate the “indirect” striatopallidal pathway to affect motor activity. At the neurochemical level, activation of the A2AAR reduces the binding affinity of D2 receptors in the striatum,21 and antagonizes many neurochemical and cellular changes brought about by the activation of striatal D2 receptors, including release of acetylcholine and GABA and expression of c-Fos. Furthermore, behavioral studies have demonstrated that the unselective adenosine antagonists caffeine and theophylline stimulate locomotor activity22,23 whereas the unselective agonist NECA24 inhibits spontaneous locomotor activity as well as motor activity induced by dopamine agonists. Thus, A2AAR agonists and antagonists function as dopamine antagonists and agonists, respectively, in modulating motor activity. The three possible mechanisms have been proposed to explain for motor enhancement by the A2A antagonists: (1) a direct receptor-receptor (A2A-D2) antagonistic interaction,25,26 (2) an opposing but independent of A2A and D2 receptor signaling27-29 or (3) A2AAR modulation of GABA release in the basal ganglia.30-32 These receptors also form A2AAR-D2 heterodimers,33 but how dimerization affects receptor function is unclear.
A2A Receptor Antagonists May Offer Multiple Therapeutic Benefits for PD Patients
First, A2A antagonists stimulate motor activity in normal as well as dopamine-depleted animals. In rodent models of PD, unselective adenosine antagonists (caffeine and theophylline)22,34 and the A2AAR-selective antagonists SCH58261, KW6002 and CSC can reverse motor deficits induced by MPTP, 6-hydroxydopamine, haloperidol or reserpine35-41 as well as by genetic deletion of D2 receptors.42 More recently, the A2A antagonists KW6002 reversed motor deficit in MPTP-treated non-human primates.43,44 Furthermore, A2A antagonists can stimulate motor activity when combined with sub-threshold doses of dopaminergic agents such as L-dopa or D1 and D2 agonists such as aporphormine or quinpirole.45 For example, combining KW6002 with L-dopa reduces the dose of L-dopa, thereby reducing the complications associated with L-dopa. In contrast to some non-specific adenosine antagonists or some dopamine agonists, motor stimulation was observed after acute treatment and persisted following treatment continued for 15 days.44,46,47 Thus, tolerance to the motor stimulant effect of A2A antagonists did not develop.
Second, studies of the MPTP-treated monkey model of PD revealed a novel feature of A2A antagonists, namely, stimulation of motor activity without dyskinesia.43,44,48 In contrast to L-dopa, repeated treatment with KW6002 reversed the motor deficit but did not induce dyskinesia, even in monkeys primed with L-dopa. Further, our recent findings in A2AAR knockout mice suggest that development of behavioral sensitization by chronic treatment of L-dopa requires activation of the A2AAR. Genetic inactivation of the A2A receptor attenuated L-dopa-induced rotational behavior.49 This is consistent with a recent study showing that co-administration of KW6002 with apomorphine to MPTP-treated monkeys completely abolished the development of apomorphine-induced dyskinesia.50 Further studies are warranted to explore the molecular mechanism underlying this novel aspect of A2AAR function.
Third, accumulating evidence suggests that the specific inactivation of A2AARs consistently attenuates brain damage induced by ischemia51,53 and excitoxicity,54,55 as well as in animal models of Huntington's disease56 and Alzheimer's disease.57 The neuroprotection by A2AAR antagonists has been recently extended to a rodent model of PD. Co-administration of A2AAR antagonists, such as CSC, DMPX, SCH58261 and KW6002 (but not the A1AR antagonist DPCPX) attenuated dopaminergic neurotoxicity in several neurotoxin models of PD.58 A2AAR antagonists provided not only functional protection (such as reduced dopamine content and expression of molecular markers for the dopaminergic terminals), but also reduced the loss of dopaminergic neurons in substantia nigra in both MPTP- and 6-OHDA models of PD.59,60 Likewise, knockout of A2AARs attenuated MPTP-induced dopaminergic neurotoxicity in mice.59 Together with the demonstration of neuroprotection by A2AAR antagonists against a wide range of neuronal injury models. These results raise the possibility that A2A antagonists may offer a neuroprotection, slowing or even halting degeneration of dopaminergic neurons.
Finally, in contrast to the widespread distribution of other neurotransmitter receptors, for example, glutamate receptors, the expression of the A2AAR is almost exclusively in striatum, which might allow selective modulation of dopamine-mediated motor pathways without serious side effects due to drug actions outside the basal ganglia (a serious problem for drugs such as glutamate antagonists). It is important to emphasize that ambient adenosine levels and A2AAR density are normal in PD patients,61 indicating that A2A antagonists might remain effective, even in the later stages of PD.
The prospective use of A2AR antagonists as potential neuroprotective agents against dopaminergic neuron degeneration was markedly enhanced by a May 2000 report of an epidemiological study of the relationship between caffeine and PD. Ross et al described a large prospective study with a 30-year follow-up of 8004 Japanese-American men that showed that in this population there is an inverse relationship between caffeine consumption and the risk of developing PD.62 Two other ongoing, large-cohort studies (Heath Professional Follow-up Studies and Nurse's Heath Study) involving 47,351 men and 88,565 women also showed that moderate caffeine consumption (3–5 cups/day) reduced their risk of developing PD.63 Thus, the inverse relationship of caffeine consumption and the risk of developing PD seem firmly established by these two large, prospective epidemiological studies. These results are consistent with the animal studies showing neuroprotection by A2AAR antagonists and strongly argue that A2A antagonists including caffeine may offer an opportunity to slow down or halt the degeneration of dopaminergic neurons.
Initial clinical trial results of KW6002 indicated that (20–80 mg/day) enhanced motor activity in one study and potentiated a motor stimulant effect by low (but not high) doses of L-dopa in another study.64,65 KW6002 was well tolerated and had few side effects. Unfortunately, trials with KW6002 have been stopped because this compound was found to produce a long-term toxicity in rats. Hence, there is a pressing need to develop alternative molecules that lack toxicity.
The first relatively selective A2AAR antagonists, the 8-styrylxanthines, appeared about ten years ago. This class includes KW-6002, which has low nanomolar affinity for the A2AAR and >100-fold selectivity for the A2AAR over the A1AR. KW-6002, entered clinical trials in 2002 as an agent for the treatment of PD.1,66 SCH58261, a pyrazolo[4,3-e]-1,2,4triazolo[1,5-c]pyrimidine was a prototype for a series of second-generation derivatives that appeared over the next several years. These, too, had low nanomolar affinity and good selectivity for the A2AAR in vitro.67 The third class of antagonists to appear, the 1,2,4-triazolo[4,5-e]-1,3,5-triazines, was typified by ZM241385, which was active at the A2AAR in the sub-nanomolar range but less selective, interacting with A2BAR as well.68 These potent A2A antagonists have been important research tools, greatly facilitating pharmacological investigations of A2AAR function in vitro as well as in vivo significantly enhancing our understanding of the neurobiology of the A2AAR. However, each of these antagonists has important drawbacks. KW-6002 is light-sensitive, undergoing photoisomerization from the active E-isomer to the 800-fold less active Z-isomer.69 SCH58261 is very poorly soluble and even its second-generation derivatives have marginal bioavailability.70 As mentioned above, ZM241385 is unselective and, additionally, has poor bioavailability. Other nitrogen heterocycles such as the 1,2,4-triazolo[4,3-a]quinoxalin-1-ones71 and the oxazolo[4,5-d]pyrimidines from ICI are also unselective, and their bioavailability is unknown. Therefore a continuing need exists for compounds that are selective A2A AR antagonists.